The biggest problem in the U.S. now is corruption. The public know this, and are therefore unprecedentedly cynical about their government; they (as will soon be documented here from a Gallup survey) overwhelmingly view our government as being corrupt. However, conservatives accept corruption as the natural order of things, something that must simply be accepted, because the rich have the most property to protect and therefore (in the view of conservatives) the rich have the right to rule so as to protect their property (since they have the most of it). Furthermore, conservatives think that the rich have earned their wealth by selling what people want, and have therefore already proven their superiority -- they've earned their control over the government. In the view of conservatives, poor people have the least property to protect, and should therefore have the least say in government. The poor are also failures economically; nobody wants to be poor; and so conservatives are doubly favorable towards rule by the rich. However, conservatives rarely vote Democratic; so, the people who don't mind our government's corruption aren't actually prospective voters for the Democratic Presidential nominee in 2016 anyway. The Democratic Party thus should simply ignore those voters, because they belong, unalterably, to the Republican Party.
Non-conservatives ("liberals"), however, don't think that the only role of government is to protect wealth; so, since Democrats are overwhelmingly not conservatives, they overwhelmingly do find disturbing that their government is corrupt, and they are therefore much more disinclined to vote for a corrupt person than non-Democrats are. Thus, if the Democratic Party were to nominate a corrupt person to represent the Party in the 2016 Presidential election, voter-turnout for the Democrat against the Republican would be significantly depressed by that fact. The Republican Party can safely nominate a corrupt person (it won't depress their vote), but the Democratic Party simply cannot safely do that. For example, Barack Obama wasn't clearly corrupt until he became President; if the public had known back then that he's corrupt, John McCain might have beaten him, instead of having been beaten by him. Hillary Clinton has a clearly corrupt record, but Barack Obama, back then, simply did not. Obama's record was ambiguous. This was crucial to Obama's victory.
Now will be presented the latest of the many surveys that show that the U.S. Government is widely recognized by the American people to be corrupt: A Gallup poll issued on 18 October 2013, was headlined "Government Corruption Viewed as Pervasive Worldwide," and Gallup buried near the end of it in a table (and they didn't even make note of the fact) showing that among the 129 countries that they surveyed, each of which nation had over a thousand citizens answering their poll in each given country, the United States was viewed by its citizens as being even a bit more corrupt than the people elsewhere in the world viewed their own country. 73% of Americans said "Yes" when asked: "Is corruption widespread throughout the government in the United States?" The people in only 62 other nations answered "Yes" at an even higher rate than 73%, so citizens in the U.S. are actually slightly more cynical about our government than citizens worldwide are about theirs. (Gallup also reported that countries that had a controlled press weren't able to fool their publics, who recognized their government's corruption notwithstanding the controlled press' trying to hide it. Corruption-perceptions were unaffected by press freedom or lack thereof.)
Although Republicans and other conservatives might not be terribly disturbed that the U.S. Government is corrupt (and they therefore aren't so opposed to "corporate lobbyists," the Supreme Court's Republican-majority "Citizens United" decision in 2010, etc.), the people who vote for a Democratic Presidential nominee are very disturbed by our government's corruption; and those people will be turned off to a Democratic Presidential nominee if that nominee becomes exposed, during the 2016 Presidential campaign, to have a clear record of corruption. Republicans won't be voting for the Democratic nominee anyway, but non-Republicans will at least consider voting for that person; and, if that nominee becomes exposed during the campaign to have a long record of corruption, then the turnout of voters for that person will be significantly reduced on Election Day, because non-Republicans do care, a lot, about whether or not a particular nominee is corrupt. Although Republicans might not be disturbed at all to know that a political candidate is controlled by the super-rich (and Romney's voters certainly did not care), non-Republicans will be very disturbed if they find that a Democratic candidate is corrupt. These voters will be disheartened to know that both of the major parties' Presidential candidates are corrupt. Especially after America's experience of the 2008 collapse, the TARP, and the lies by the G.W. Bush Administration about "Saddam's WMD," etc., the public's finding out that both parties have corrupt nominees to become President will depress the Democratic vote far more than it will depress the Republican vote.
Consequently, it makes no sense at all for a Democrat who has an extensive and incontrovertible record of corruption to be chosen by Democratic voters in the 2016 primaries to become the person who will represent the Democratic Party in the general election contest against the Republican nominee. If Democratic primary voters don't learn of their nominee's corruption before the primaries, then the Democratic Party could well end up being stuck, in the general election, with a nominee who will be extremely vulnerable to the Republican Party's exposure of that person's corrupt record, during the general-election campaign.
Hillary Clinton's corruption thus far has not been exposed by the Republican Party, because they've had no need to do so: she hasn't been the Democratic nominee for President. But there is also another important reason:
The Republican Party is naturally reluctant to expose a Democratic politician's corruption if the chief beneficiaries of it have been major Republican donors. For example, Wall Street has benefited enormously from Obama's protection; like he told Wall Street's chieftains in a private meeting at the White House, on 27 March 2009, "My administration ... is the only thing between you and the pitchforks." Obama was courting there the very same people who had donated a far higher percentage of the McCain campaign's cash than they had donated of the Obama campaign's cash; Obama might have been aiming to pull more of their money his way next time; but, in 2012, Wall Street went overwhelmingly for Romney, because Romney was offering them an even better deal than Obama did. Republicans won't accuse Obama of corruption that benefits their own big donors. For example, how would the Republican Party have been able to attack Obama for his corruption, if their own nominee, Romney, was an even bigger sell-out to those very same donors? Consequently, the Republican Party has instead accused Obama only of alleged "corruption" to environmental industries, such as solar-power firms, irrespective of whether any corruption at all was actually involved there.
It is likely to be different if Hillary Clinton becomes the Democratic nominee. In one important respect, however, she is just like Obama: her corruption has been chiefly in service to big Republican donors. Ever since her husband entered the White House in 1993 with that strategy, it has been the dominant strategy for major Democratic Presidential aspirants; Hillary's corruption in shaping her health-care plan to benefit the private HMO industry, and Bill's corruption shaping his deregulation of Wall Street to benefit Citigroup and other Wall Street titans, has been a winning political strategy. Obama has merely been doing what Hillary would have been doing if she had become the President. In that sense, they are virtually the same. But here is how the situation would be different if Hillary Clinton would become the Democratic Party's nominee for the Presidency in 2016: